1. Field of the Invention
The present invention relates to an electronic device and a method of driving an electronic device. In particular, the present invention relates to an active matrix electronic device having thin film transistors formed on an insulator, and to an active matrix electronic device using the method of driving an electronic device. Among such devices, the present invention relates to an active matrix device using a digital image signal as an image source and using a self light emitting element such as an organic electro luminescence (EL) element (a light emitting diode or OLED (Organic Light Emission Diode)) in a pixel portion, and to an active matrix electronic device using the method of driving. The EL devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example.
2. Description of the Related Art
The spread of electronic devices having a semiconductor thin film formed on an insulator such as a glass substrate, in particular active matrix electronic devices using thin film transistors (hereafter referred to as TFTs), has become remarkable in recent years. Active matrix electronic devices using TFTs have from several hundred thousand to several million TFTs arranged in a matrix shape, and display of an image is performed in accordance with controlling the electric charge of each pixel.
In addition, techniques relating to polysilicon TFTs in which pixel TFTs structuring pixels, and driver circuits using TFTs in the peripheral of a pixel portion, are formed at the same time have been developed recently. The miniaturization of the devices has contributed greatly to lowering their electric power consumption, and in accordance with their low power consumption and miniaturization, the electronic devices have become indispensable devices in portions such as display portions of mobile devices, an application which has exploded in recent years.
Further, research is enthusiastically being performed and all eyes are focused on electronic devices applying self-light emitting materials, such as organic EL materials, as a flat panel display substitute for LCDs (liquid crystal displays).
An example of a schematic diagram of a digital electronic device is shown in FIG. 13. A pixel portion 1307 is placed in the center. Source signal lines, gate signal lines, and in addition, an electric current supply line 1306 for supplying electric current to EL elements are arranged in the pixel portion 1307. A source signal line driver circuit 1301 is placed on the upper side of the pixel portion 1307 in order to control the source signal lines. The source signal line driver circuit 1301 has circuits such as shift register circuits 1303, first latch circuits 1304, and second latch circuits 1305. On the left and right of the pixel portion 1307 are arranged gate signal line driver circuits 1302 for controlling the gate signal lines. Note that although the gate signal line driver circuits 1302 are arranged on both left and right sides of the pixel portion 1307 in FIG. 13, they may also be placed on only one side. However, from a standpoint of driving efficiency and driving reliability, it is preferable to place them on both sides.
The source signal line driver circuit 1301 has a structure like that shown in FIG. 14, and has circuits such as shift register circuits (SR) 1401, first latch circuits (LAT1) 1402, and second latch circuits (LAT2) 1403. Note that, although not shown in FIG. 14, circuits such as buffer circuits and level shifter circuits may also be used when necessary.
The operation is explained simply using FIG. 13 and FIG. 14. First, clock signals (S-CLK and S-CLKb), and start pulses (S-SP) are input to the shift register circuit 1303 (denoted by SR in FIG. 14), and pulses are output one after another. Subsequently, the pulses are input to the first latch circuits 1304 (denoted by LAT1 in FIG. 14), and a digital image signal (digital data) input to the first latch circuits 1304 is stored. When the storage of each one bit portion of the digital image signal is completed during one horizontal period in the first latch circuit 1304, the digital image signal stored by the first latch circuit 1304 within a return period is transferred all at once to the second latch circuits 1305 (denoted by LAT2 in FIG. 14) in accordance with the input of a latch signal (latch pulse).
On the other hand, gate side clock signals (G-CLK) and gate side start pulses (G-SP) are input to shift registers (not shown in the FIGS.) in the gate signal line driver circuits 1302. The shift registers output pulses one after another based on the input signals, and these are output as gate signal line selection pulses through circuits such as buffers (not shown in the figures), and the gate signal lines are selected in order.
The data transferred to the second latch circuits 1305 of the source signal line driver circuit 1301 is then written into a column of pixels selected in accordance with the gate signal line selection pulse.
Driving operation of the pixel portion 1307 is explained next. A portion of the pixel portion 1307 is shown in FIGS. 19A and 19B. FIG. 19A shows a 3×3-pixel matrix, and a portion contained with a dotted line frame 1900 is one pixel. A blowup diagram of one pixel is shown in FIG. 19B. Reference numeral 1901 in FIG. 19B denotes a TFT which functions as a switching element when writing a signal into the pixel (hereafter referred to as a switching TFT). Either an n-channel polarity or a p-channel polarity may be used for the switching TFT 1901. Reference numeral 1902 denotes a TFT (hereafter referred to as an EL driver TFT), which functions as an element (electric current control element) in order to control electric current supplied to an EL element 1902. If a p-channel TFT is used for the EL driver TFT 1903, then it is arranged between an anode 1909 of the EL element 1903 and an electric current supply line 1907. As another, different structuring method, it is also possible to arrange the EL driver TFT 1902 between a cathode 1910 of the EL element 1903 and a cathode electrode 1908 if an n-channel TFT is used as the EL driver TFT 1902. However, a method in which the EL driver TFT 1902 is arranged between the anode 1909 of the EL element 1903 and the electric current supply line 1907 is general and often employed due to such factors as source grounding for TFT operation and manufacturing restrictions on the EL element 1903. Reference numeral 1904 denotes a storage capacitor in order to store a signal (voltage) input from the source signal line 1906. One terminal of the storage capacitor 1904 is connected to the electric current supply line 1907 in FIG. 19B, but a specialized wiring may also be used. A gate electrode of the switching TFT 1901 is connected to a gate signal line 1905, and a source region of the switching TFT 1901 is connected to the source signal line 1906.
Operation of active matrix electronic device circuits is explained next with reference to the same FIGS. 19A and 19B. First, a voltage is applied to the gate electrode of the switching TFT 1901 when the gate signal line 1905 is selected, and the switching TFT 1901 is placed in a conductive state. The signal (voltage) of the source signal line 1906 is stored in the storage capacitor 1904 by doing so. The voltage of the storage capacitor 1904 becomes a voltage VGS between a gate and a source of the EL driver TFT 1902, and therefore an electric current corresponding to the voltage of the storage capacitor 1904 flows in the EL driver TFT 1902 and the EL element 1903. The EL element 1903 turns on as a result.
The brightness of the EL element 1903, namely the amount of electric current flowing in the EL element 1903, can be controlled in accordance with VGS of the EL driver TFT 1902. VGS is the voltage of the storage capacitor 1904, and that is the signal (voltage) input to the source signal line 1906. In other words, the brightness of the EL element 1903 is controlled by controlling the signal (voltage) input to the source signal line 1906. Finally, the gate signal line 1905 is placed in an unselected state, the gate of the switching TFT 1901 is closed, and the switching TFT 1901 is placed in an unselected state. The electric charge, which has accumulated in the storage capacitor 1904, is maintained at this point. VGS of the EL driver TFT 1902 is therefore maintained as it is, and the amount of electric current corresponding to VGS continues to flow in the EL element 1903 via the EL driver TFT 1902.
Information regarding EL element drive is reported upon in papers such as the following: “Current Status and Future of Light-emitting Polymer Display Driven by Poly-Si TFT”, SID99 Digest, p. 372; “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, ASIA DISPLAY 98, p. 217; and “3.8 Green OLED with Low Temperature Poly-Si TFT”, Euro Display 99 Late News, p. 27.
A method of gray scale display in an EL element is discussed next. An analog method of the gray scale display has a disadvantage in that it is weak with respect to dispersion in the electric current characteristics of the EL driver TFTs. Namely, if the electric current characteristics of the EL driver TFTs differ, then the value of electric current flowing in the EL driver TFTs and the EL elements changes even if the same gate voltages are applied. As a result, the EL element brightness, namely the gray scale, also changes.
A method referred to as a digital gray scale method has therefore been proposed in order to reduce the influence of dispersion in the characteristics of the EL driver TFTs. This method is a method for controlling the gray scale by two states, a state in which the absolute value of the gate voltage |VGS| of the EL driver TFT is below the turn on start voltage (in which almost no electric current flows), and a state in which the absolute value of the gate voltage |VGS| is greater than the brightness saturation voltage (in which an electric current close to the maximum flows). In this case, the value of the electric current becomes close to IMAX even if there are dispersion in the electric current characteristics of the EL driver TFTs, provided that the absolute value of the gate voltage |VGS| of the EL driver TFT is sufficiently larger than the brightness saturation voltage. The influence of EL driver TFT dispersions can therefore be made extremely small. The gray scales are thus controlled into an ON state (bright state due to maximum electric current flow) and an OFF state (dark state due to no electric current flow). This method is therefore referred to as a digital gray scale method.
However, only two gray scales can be displayed with the digital gray scale method. A plurality of techniques which can achieve multiple gray scales, in which another method is combined with the digital gray scale method, have been proposed.
A time gray scale method is one method that can be used to achieve multiple gray scales. The time gray scale method is a method in which the time during which the EL elements are turned on is controlled, and gray scales are output by the length of the turn on time. In other words, one frame period is divided into a plurality of subframe periods, and gray scales are realized by controlling the number and the length of the subframe periods during which turn on is performed.
Refer to FIGS. 9A to 9D. Drive timing for a circuit using a time gray scale method is shown simply in FIGS. 9A to 9D. An example of obtaining 3-bit gray scales by a time gray scale method with the frame frequency set to 60 Hz is shown.
As shown in FIG. 9A, one frame period is divided into a number of subframes corresponding to the number of gray scale bits. Three bits are used here, and therefore one frame period is divided into three subframes. One subframe period is further divided into an address period (Ta) and a sustain (turn on) period (Ts). (See FIG. 9B.) A sustain period during a subframe period, which is denoted by reference symbol SF1, is referred to as Ts1. Similarly, sustain periods for the cases of subframes SF2 and SF3 are referred to as Ts2 and Ts3, respectively. Address periods are periods during which one frame portion of an image signal is written into the pixels, and therefore the lengths of the address periods are equal in all of the subframe periods. (See FIG. 9C.) The sustain periods have lengths proportional to powers of 2, and the sustain periods here are such that Ts1:Ts2:Ts3=22:21:20=4:2:1.
As a gray scale display method, the brightness is controlled by the length of the sum of all turn on periods within one frame period in accordance with controlling to set the EL elements either to a turned on state or a turned off state, in the sustain (turn on) periods from Ts1 to Ts3. In this example, 23=8 turn on time lengths can be set by combining the sustain (turn on) periods, and therefore 8 gray scales can be displayed. Gray scales are thus expressed by utilizing the length of the turn on time.
In addition, the number of divisions within one frame period may also be increased for a case of increased gray scales. The proportional lengths of the sustain (turn on) periods for a case of dividing one frame period into n subframe periods becomes Ts1:Ts2: . . . :Ts(n−1):Tsn=2(n−1):2(n−2): . . . :21:20, and it becomes possible to express 2n gray scales.
In order to make dynamic display smooth in a general active matrix electronic device, renewal of the image display is performed approximately 60 time during one second, as shown in FIG. 9A. In other words, the digital image signal is supplied in every frame and it is necessary to perform write in to the pixels each time. For example, even if an image is static, the same signal must be supplied during every single frame, and therefore it is necessary for the driver circuit to operate continuously and to repeatedly process the same digital image signal.
There is also a method of once writing in a static digital image signal to an external memory circuit, and subsequently supplying the digital image signal from the external memory circuit during every single frame, but even in this case there is no change in the necessity for the external memory circuit and the driver circuit to operate continuously.
In particular, it is preferable that a mobile device has greatly reduced electric power consumption. In addition, even though a static picture mode occupies large portions with a mobile device, as stated above, circuits such as driver circuits continue to operate when displaying a static image, and this stops reductions in electric power consumption.